In a typical electrostatographic reproducing apparatus, a light image of an original to be copied is recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles which are commonly referred to as toner. The visible toner image is then in a loose powdered form and can be easily disturbed or destroyed. The toner image is usually fixed or fused upon a support which may be the photosensitive member itself or other support sheet such as plain paper.
The use of thermal energy for fixing toner images onto a support member is well know. In order to fuse electroscopic toner material onto a support surface permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to be firmly bonded to the support.
Typically, the thermoplastic resin particles are fused to the substrate by heating to a temperature of between about 90.degree. C. to about 160.degree. C. or higher depending upon the softening range of the particular resin used in the toner. It is undesirable, however, to raise the temperature of the substrate substantially higher than about 200.degree. C. because of the tendency of the substrate to discolor at such an elevated temperatures particularly when the substrate is paper.
Several approaches to thermal fusing of electroscopic toner images have been described in the prior art. These methods include providing the application of heat and pressure substantially concurrently by various means: a roll pair maintained in pressure contact; a belt member in pressure contact with a roll and the like. Heat may be applied by heating one or both of the rolls, plate members or belt members. The fusing of the toner particles takes place when the proper combination of heat, pressure and contact time are provided. The balancing of these parameters to bring about the fusing of the toner particles is well known in the art, and they can be adjusted to suit particular machines or process conditions.
During operation of a fusing system in which heat is applied to cause thermal fusing of the toner particles onto a support, both the toner image and the support are passed through a nip formed between the roll pair, or plate or belt members. The concurrent transfer of heat and the application of pressure in the nip effects the fusing of the toner image onto the support. It is important in the fusing process that no offset of the toner particles from the support to the fuser member takes place during normal operations. Toner particles offset onto the fuser member may subsequently transfer to other parts of the machine or onto the support in subsequent copying cycles, thus increasing the background or interfering with the material being copied there. The so called "hot offset" occurs when the temperature of the toner is raised to a point where the toner particles liquefy and a splitting of the molten toner takes place during the fusing operation with a portion remaining on the fuser member. The hot offset temperature or degradation of the hot offset temperature is a measure of the release property of the fuser roll, and accordingly it is desired to provide a fusing surface which has a low surface energy to provide the necessary release. To insure and maintain good release properties of the fuser roll, it has become customary to apply release agents to the fuser members to insure that the toner is completely released from the fuser roll during the fusing operation. Typically, these materials are applied as thin films of, for example, silicone oils to prevent toner offset.
Some recent developments in fuser members, release agents and fusing systems are described in U.S. Pat. No. 4,264,181 to Lentz et al., U.S. Pat. No. 4,257,699 to Lentz and U.S. Pat. No. 4,272,179 to Seanor, all commonly assigned to the assignee of the present application. These patents describe fuser members and methods of fusing thermoplastic resin toner images to a substrate wherein a polymeric release agent having functional groups is applied to the surface of the fuser member. The fuser member comprises a base member having an elastomeric surface with a metal containing filler therein which has been cured with a nucleophilic addition curing agent. Exemplary of such fuser member is an aluminum base member with a poly(vinylidenefluoride-hexafluoropropylene) copolymer cured with a bisphenol curing agent and having lead oxide filler dispersed therein and utilizing a mercapto functional polyorganosiloxane oil as a release agent. In those fusing processes, the polymeric release agents have functional groups (also designated as chemically reactive functional groups) which interact with the metal containing filler dispersed in the elastomer or resinous material of the fuser member surface to form a thermally stable film which releases thermoplastic resin toner and which prevents the thermoplastic resin toner from contacting the elastomer material itself. The metal oxide, metal salt, metal alloy or other suitable metal compound filler dispersed in the elastomer or resin upon the fuser member surface interacts with the functional groups of the polymeric release agent. Preferably, the metal containing filler materials do not cause degradation of or have any adverse effect upon the polymeric release agent having functional groups. Because of this reaction between the elastomer having a metal containing filler and the polymeric release agent having functional groups, excellent release and the production of high quality copies are obtained even at high rates of speed of electrostatographic reproducing machines.
While the mechanism involved is not completely understood, it has been observed that when certain polymeric fluids having functional groups are applied to the surface of a fusing member having an elastomer surface with a metal oxide, metal salt, metal, metal alloy or other suitable metal compounds dispersed therein there is an interaction (a chemical reaction, coordination complex, hydrogen bonding or other mechanism) between the metal of the filler in the elastomer and the polymeric fluid having functional groups so that the polymeric release agent having functional groups in the form of a liquid or fluid provides an excellent surface for release having an excellent propensity to remain upon the surface of the fuser member. Regardless of the mechanism, there appears to be the formation of a film upon the elastomer surface which differs from the composition of the elastomer and the composition of the polymeric release agent having functional groups. This film, however, has a greater affinity for the elastomer containing a metal compound than the toner and thereby provides an excellent release coating upon the elastomer surface. The release coating has a cohesive force which is less than the adhesive forces between heated toner and the substrate to which it is applied and the cohesive forces of the toner. The interaction between the functional group of the polymeric release agent and the metal of the elastomer containing metal leads to an overall diminution of the critical or high surface energy of the metal in the metal containing filler.
The use of polymeric release agents having functional groups which interact with a fuser member to form a thermally stable, renewable self-cleaning layer having superior release properties for electroscopic thermal plastic resin toners is described in U.S. Pat. No. 4,029,827 to Imperial et al., U.S. Pat. No. 4,101,686 to Strella et al. and U.S. Pat. No. 4,185,140 also to Strella et al. all commonly assigned to the assignee of the present invention. In particular, U.S. Pat. No. 4,029,827 is directed to the use of polyorganosiloxanes having mercapto functionality as release agents. U.S. Pat. Nos. 4,101,686 and 4,185,140 are directed to polymeric release agents having functional groups such as carboxy, hydroxy, epoxy, amino, isocyanate, thioether, and mercapto groups as release fluids. Some of these fusing systems have enjoyed significant commercial application. For example, a fuser roll made from Viton E 45 (a copolymer of 77 mole percent vinylidenefluoride and 23 mole percent hexafluoropropylene) filled with lead oxide has been successfully used in a fusing system employing a mercapto functional polyorganosiloxane release agent.
While these materials have been used successfully in several commercial applications, they nevertheless exhibit difficulties in certain applications or under certain conditions. In applications where the temperature of the fuser roll reaches 240.degree. C., difficulties are experienced with the fusing layer or release layer adhering to a steel substrate. Typically, an epoxy adhesive such as Thixon 300/301 is used between the steel substrate and the polyvinylidene fluoride hexafluoropropylene fusing surface layer which experiences adhesion failure at elevated temperature. This adhesion failure is experienced by delamination of the copolymer layer from the steel substrate within 500 hours of use leading to premature failure and replacement. Many factors effect this debonding of the copolymer fusing surface at such temperatures including the fact that the epoxy adhesive loses it binding efficiency to the metal or becomes weak and splits. In addition, at the higher temperatures, the steel has a tendency to corrode which also inhibits good bonding. Further, in certain applications, high levels of strain energy in the fusing nip between the fuser roll and the pressure roll can contribute to adhesion failure.